Automatic gain control apparatus

ABSTRACT

A playback signal from an optical disk includes a track cross component which is produced when a head traverses tracks on the disk and a recorded area component the level of which is reduced in comparison with that of an area on which no data is recorded. A control signal is generated which includes the recorded area component but not the track cross component. A gain of a tracking servo control is controlled by the control signal.

BACKGROUND OF THE INVENTION

This invention relates to an automatic gain control apparatus, includinga servo control unit responsive to a servo error signal for locating amovable head with respect to a recording medium.

Optical disk have been employed as a recording medium has been employedto store data on tracks thereof. An optical head is used to play backthe data recorded on the optical disk. A typical optical head includes alight source, such as a semiconductor laser, for radiating a light(laser) beam through an objective lens onto the optical disk. Theobjective lens is positioned at a predetermined distance from theoptical disk for focusing the light beam on the data recording layer onthe optical disk. The optical head also includes photo sensors forsensing the light beam reflected from the optical disk to determine thepresence and absence of pits formed in the spiral tracks of the opticaldisk. The objective lens is supported in such a manner that its opticalaxis can follow the spiral track and can move from one track to adesired track during a seeking operation, as well as in the direction ofthe optical axis for focusing.

In order to locate the optical head at an optimum position against theoptical disk, the optical head is associated with a focusing servo unitfor moving the objective lens in a focusing direction parallel to theoptical axis thereof so as to maintain the objective lens at thepredetermined distance from the optical disk if the distance of theobjective lens from the optical disk deviates from the predetermineddistance. The optical head is also associated with a tracking servo unitfor moving the objective lens in a tracking direction perpendicular tothe track to locate the light spot on the optical disk at the center ofthe track when the optical axis deviates from the track center.

A part of the light beam reflected on the optical disk is incident on aphoto sensor divided into four independent elements A1, B1, C1 and D1,as shown in FIG. 1A. Another part of the reflected light beam is splitinto two light beams incident on respective two photo sensor elements E2and F2, located at positions corresponding to the opposite sides of thetrack, at the same distance from the track center, as shown in FIG. 1B.Each photo sensor element converts the light incident thereon into anelectric signal having a level corresponding to the intensity of theincident light beam. The electric signals derived from photo sensorelements A1 and C1 are added to form an FA signal. The electric signalsderived from the photo sensor elements B1 and D1 are added to form an FBsignal. The photo sensor element E2 converts the light incident thereoninto a TA signal having a level corresponding to the intensity of theincident light, and the photo sensor element F2 converts the lightincident thereon into a TB signal having a level corresponding to theintensity of the incident light. The playback signals derived from thephoto sensor elements A1-D1, E2 and F2 are added to form an RF signal.The RF signal is to read data recorded on the optical disk, but the TAand TB signals generate a tracking error signal and the latter are notrequired to include data components recorded on the optical disk.Therefore the TA and TB signals are limited to a narrow band offrequencies (e.g., below 50 KHz).

Each track comprises a plurality of sectors, each having a pre-formatarea preceding to a user area on which pits are formed to store datafrom, for example a music or image source. The pre-format area haspre-format data previously recorded thereon for use in recording andplaying back data on the user area.

FIG. 2A shows a waveform of a pre-format signal included in the RFsignal resulting from playback of the pre-format data recorded on thepre-format area under a tracking servo control. The pre-format signalincludes a sector mark (SM), a mirror mark (ODF) and VFO and ID signalsrepeated three times alternatively between the sector mark (SM) and themirror mark (ODF). The sector mark indicates the start of the pre-formatsignal. Each VFO signal contains clock pulses required to reproduce thesucceeding ID signal and has a frequency higher than any other signalsrecorded on the optical disk. Each ID signal includes at least a sectoraddress and an error detection code. The mirror mark (ODF) is used toadjust the electrical offset of the tracking servo circuit. The mirrormark (ODF) has a level higher than any other signals recorded on theoptical disk. A signal from a non-recorded area on the track, which isformed as a pre-groove because the ODF is not formed as a pre-groove,but by leaving a mirror surface of the optical disk.

FIG. 2B shows the RF signal on a reduced time scale. As shown in FIG.2C, the TA (or TB) signal, which is produced from the photo sensorelement E2 (or F2), has a level that is lower when the optical headpasses the pre-format area because the pre-format data is recorded onthe pre-format area. However, the data recorded on the optical disk cannot be played back from the TA (or TB) signal.

FIG. 2D shows the RF signal on a further reduced time scale. It isassumed that the user area of the data sector No. N+1 has recorded datathereon, as indicated by the hatched area in FIG. 2D. As shown in FIG.2E, the level of the TA (or TB) signal is also lower at the user area ofthe data sector No. N+1, as well as the pre-format area. It is,therefore, apparent that the level of the tracking error signal producedfrom the difference (TB-TA) between the TB and TA signals is reduced toa lower level in the preformat and user areas having data recordedthereon than in the user areas having no data recorded thereon.Consequently, the tracking servo control is particularly sensitive todisturbances when the optical head passes the recorded areas.

In order to avoid this difficulty, an automatic gain control (AGC)circuit is required to compensate for the level drop of the trackingerror signal in the recorded areas. In previous apparatus, the gaincontrol is performed on an assumption that the level drop of thetracking error signal can be represented merely by the correspondinglevel drop in the RF signal. Therefore, it was very difficult to providean accurate compensation for the level drop of the tracking error signalbecause track cross components are superimposed in the (TA+TB) signaland the tracking error signal (TB-TA), as shown in FIGS. 2F and 2G,respectively, when the optical head traverses the tracks with thetracking servo control being suspended. Such a gain control will renderit difficult to ensure a rapid lock-in operation when the tracking servocontrol is resumed. In addition, the counter, which is used to count thenumber of peaks of the RF signals which correspond to the number oftraversed tracks, would accumulate an incorrect count.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide animproved automatic gain control apparatus which can perform an accurateservo control that is free from the influence of a track crosscomponent.

There is provided, in accordance with this invention, an automatic gaincontrol apparatus comprising a first means for generating at least onetracking error signal and focusing error signal for a tracking andfocusing servo to control a relative position of a head against arecording medium, which has tracks on which information is recorded, inthe tracking and focusing directions, respectively;

a second means for controlling a gain of at least one of said errorsignals in accordance with a control signal;

a third means for generating a compensated playback signal bycompensating the level of a first signal which has a predeterminedfrequency and is included in a playback signal from said recordingmedium; and

a fourth means for generating said control signal from said compensatedplayback signal when said tracking servo is on.

According to this invention, a rapid lock-in operation can be performed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams showing photo sensor elementsused in an optical head;

FIG. 2 shows waveforms of playback signals produced from the opticalhead;

FIG. 3 is a circuit diagram of a servo error signal generator used in anautomatic gain control apparatus of this invention;

FIGS. 4A and 4B are diagrams used in explaining playback signal leveldrops;

FIG. 5 is a circuit diagram of a gain control signal generator used inthis invention;

FIGS. 6A and 6B are circuit diagrams of inverters used in thisinvention;

FIG. 7 is a circuit diagram of a TCR signal generator used in thisinvention;

FIG. 8 shows waveforms obtained in the TCR signal generator;

FIG. 9 shows waveforms obtained in the TCR signal generator when thetracking servo control is OFF;

FIG. 10 shows waveforms obtained in the TCR signal generator when thetracking servo control is ON;

FIG. 11 shows waveforms obtained in the TCR signal generator when thetracking servo control is OFF;

FIG. 12 shows waveforms used in explaining a mirror mark detectingoperation;

FIG. 13 shows are waveforms obtained in the TCR signal generator whenthe tracking servo control is ON;

FIG. 14 shows waveforms obtained in the TCR signal generator;

FIG. 15 shows are waveforms used in explaining the operation of theservo error signal generator; and

FIG. 16 shows a disk and movable head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 16 depicts an exemplary view of a typical combination of movablehead 104 and disk 100 having tracks 102 thereon, with which theautomatic gain control apparatus of the present invention may beimplemented.

Referring to FIG. 3, there is illustrated one embodiment of a servoerror signal generator used in an automatic gain control apparatus ofthis invention. The servo error signal generator includes an inputterminal 15 to which an FA signal is applied and another input terminal16 to which an FB signal is applied. Input terminal 15 is coupledthrough a resistor R101 to a negative input of an operational amplifier0101, while a positive input coupled through resistor R102 to the inputterminal 16. The operational amplifier 0101 forms a differentialamplifier along with resistors R103 and R104. The differential amplifierproduces an error signal (FB-FA) at the output of the operationalamplifier 0101 that is indicative of a difference between the FA and FBsignals. The output of the operational amplifier 0101 is coupled to afirst input I1 and a second input I4 of a first integrated circuit IC101a second input. The integrated circuit IC101 controls the gain of thefocusing servo by dividing the value of the signal applied to the firstinput I1 by the value of the signal applied to the second input I4 andoutputting a signal indicative of the result of the division at outputI3. Output I3 of the integrated circuit IC101 is coupled to the negativeinput of an operational amplifier O103 that has a grounded positiveinput. The operational amplifier O103 forms an amplifier in conjunctionwith resistors R119, R120 and R121. A focusing error signal produced atthe output of the operational amplifier O103 is supplied to an outputterminal 42 for connection to a focusing servo control circuit (notshown).

With reference to FIG. 3B, the servo error signal generator alsoincludes an input terminal 17 to which the TA signal is applied andanother input terminal 18 to which the TB signal is applied. The inputterminal 17 is coupled through resistor R131 to a negative input of anoperational amplifier O108, while a positive input of the operationalamplifier Q108 is coupled through resistor R133 to the input terminal18. The operational amplifier O108 forms a differential amplifier alongwith resistors R132 and R134. The differential amplifier produces anerror signal (TB-TA) that is indicative of a difference between the TAand TB signals from the output of operational amplifier O108. The outputof operational amplifier O108 is coupled through resistor R140 to afirst input I1 and a second input I4 of a second integrated circuitIC102. The second integrated circuit IC102 controls the gain of thetracking servo by dividing the value of the signal applied to the firstinput I4 by the value of the signal applied to the second input I4 andproducing an output signal at output I3. Output I3 of the secondintegrated circuit IC102 is coupled to a negative input of operationalamplifier O106 that has a grounded positive input. The operationalamplifier O106 forms an amplifier, along with resistors R149, R155 andR151. A tracking error signal produced at the output of the operationalamplifier O106 is coupled to an output terminal 43 for connection to atracking servo control circuit (not shown).

Since the optical disk is usually a constant angular velocity (CAV) typedisk, the length of pits formed in recording a signal on an optical diskdecreases as the pits are formed closer to the center of the opticaldisk and also as the signal has a higher frequency. FIG. 4A shows thewaveforms of an RF signal played back from an inner track, whereas FIG.4B shows the waveforms of the RF signal played back from an outer track.As can be seen from a comparison of FIGS. 4A and 4B, the levels of a VFOsignal played back from the inner track are lower than that of a VFOsignal played back from the outer track because each VFO signal has ahigh frequency.

With reference to FIG. 5, there is illustrated a gain control (GC)signal generator used in the apparatus of the present invention forcompensating for a VFO signal level drop. The gain control signalgenerator includes a first input terminal 51 to which a -GSP signal isapplied from a computer 50 and a second input terminal 52 to which atrack position signal is applied from the computer 50. The level of the-GSP signal is inverted to a logic level L, for example each time theoptical head crosses a predetermined number of tracks. Input terminal 51is coupled through two inverters I601 and I602 to a gate of a fieldeffect transistor (FET) Q601.

With reference to FIGS. 6A and 6B, inverter I601 comprises a NPNtransistor Q1 the base of which is connected with an input terminalthrough a resistor R1 and with the grounded emitter thereof through aresistor R2 and the collector of which is connected to an outputterminal. Therefore, inverter I601 outputs a logic level L when a logiclevel H is inputted and the output thereof is OPEN when a logic level Lis inputted. Inverter I602 comprises a PNP transistor Q2 the base ofwhich is connected with an input terminal through a resistor R3, theemitter of which is connected to a specific voltage and with the basethereof through a resistor R4 and the collector of which is connected toan output terminal. Therefore, the inverter I602 outputs a logic level Hwhen a logic level L is inputted and the output thereof is OPEN when alogic level H is inputted.

The track position signal is indicative of the distance at which theoptical head is located in a radial direction of the optical disk. Theinput terminal 52 is coupled to resistor R601 and capacitor C601, whichforms an integrator. The output of the integrator is coupled throughresistor R602 to a positive input of an operational amplifier O601 thatfunctions as a buffer amplifier. The output of the operational amplifierO601 is coupled through resistor R603 to a drain of FET Q601, while asource of the FET Q601 is grounded through capacitor C602. The FET Q601is turned ON to charge capacitor C602 with the output of the operationalamplifier O601 when the -GSP signal has a LOW level. The FET Q601 isturned OFF to disconnect the capacitor C602 from the output of theoperational amplifier O601 when the -GSP signal has a HIGH level. Thesource of FET Q601 is coupled to an output terminal 38 through anamplifier which comprises operational amplifiers O602 and O603 and anFET Q602 to produce a gain control (GC) signal at the output terminal38. Thus, the gain control (GC) signal has a signal level that is incompliance with a position on the optical head in the radial directionof the optical disk.

With reference to FIG. 7, a played back RF signal is applied to an inputterminal 31 and is divided into two signal paths. The first signal pathincludes a capacitor C501, an FET Q501, resistors R502 and R503 and isconnected to a negative input of an operational amplifier O501. Thesecond signal path includes a capacitor C503 and a band pass filter,including a resistor R504, inductance L501 and capacitor C504. This bandpass filter has a center frequency equal to the frequency of the VFOsignal to extract the VFO signal from the RF signal. The output of theband pass filter is coupled to the base of transistor Q502, having anemitter connected to a source of FET Q503 which has a drain connectedthrough resistor R508 to a specific voltage and a gate connected to thespecific voltage. The transistor Q502 forms a negative amplifier forinverting and amplifying the extracted VFO signal at an amplificationdegree determined by a current flow through the emitter of transistorQ502. The inverted and amplified VFO signal appears on the collector oftransistor Q502 and is applied through a capacitor C505 to a positiveinput of the operational amplifier O501.

The emitter of transistor Q502 is also connected through capacitor C506to terminal 38 to which the gain control (GC) signal is applied from thecircuit shown in FIG. 5. Terminal 38 is also connected to an anode of aPIN diode D502, having a cathode grounded through capacitor C507 andalso to a specific voltage through resistor R508. The PIN diode D502 hasa variable resistance that is dependent on the level of the gain control(GC) signal supplied from terminal 38. Therefore, the level of the gaincontrol (GC) signal has a direct effect on the impedance of the circuitconnected to the emitter of transistor Q502, and thus on theamplification degree of the transistor Q502. The amplification degree ofthe transistor Q502 increases as the optical head is positioned closerto the center of the optical disk. The operational amplifier O501 formsa negative differential amplifier for producing a differential signal atthe output thereof. The differential signal is indicative of thedifference between the RF signal inputted from terminal 31, as shown inFIG. 8A, and the VFO signal amplified at an amplification degreedetermined by the gain control (GC) signal to compensate for the VFOsignal level drop related to the radial position of the optical head.FIG. 8B shows the waveform of the compensated signal produced at theoutput of the operational amplifier O501 in an inverted form incomparison with the RF signal. The differential signal is applied fromthe output of operational amplifier O501 to an output terminal 33 forconnection to a reading circuit (not shown), where the played backpre-format signal is read.

Capacitor C501 is connected through a series connection of a diode D501and resistor R501 to the output of operational amplifier O501. Since theRF signal shown in FIG. 8A is applied to the negative input of theoperational amplifier O501, the RF signal is inverted by it. The diodeD501 is forward biased to charge the capacitor C501 with the played backRF signal fed thereto from the input terminal 31 only when a mirror mark(ODF) having the highest level is played back. When no mirror mark (ODF)is played back, the diode D501 is reverse biased. In this case,capacitor C501 is discharged through a line including resistors R502,R503 and R504 which is connected to the output of operational amplifierO501. Thus, the operational amplifier O501 produces an output signalwherein the level of each mirror mark (ODF) is clamped at apredetermined potential, as shown in FIGS. 9B and 10B. This is effectiveto maintain the played back RF signal in a predetermined range in spiteof any mirror mark level variations caused by variations in thereflection factor of the mirror surface portion of the optical disk.

The time constant, which determines the discharging time of thecapacitor C501, is set at a desired value to prevent distortions of theoutput of the operational amplifier O501, as shown in FIGS. 9C and 10C,and to maintain the reflection factor variations in the respectivesectors within a permissible range. Resistor R501 is effective to keepthe waveform of the output of operational amplifier O501 free fromdistortion during the charging duration of the capacitor C501. Althoughthe distortion avoiding effect is greater with resistor R501 having agreater resistance, the response speed will be slower since the timeconstant increases. For this reason, the resistance of resistor R501 isset at an appropriate value.

The output of operational amplifier O501 is also coupled to a bandelimination filter having an inductance L511, a capacitor C524 and aresistor R550. This band elimination filter has a center frequencysubstantially equal to that of the VFO signal included in the RF signalfor removing the VFO signal from the RF signal fed thereto from theoperational amplifier O501. This is effective to avoid an incorrectmirror mark detection by mistaking the VFO signal for the mirror mark(ODF). The output of the band elimination filter is coupled to a base oftransistor Q508, having an emitter connected through resistor R549 to aspecific voltage and a collector connected to another specific voltage.The emitter of transistor Q508 is also connected through a capacitorC523 to a gate of FET Q507. FET Q507 has a source coupled to a specificvoltage and a drain connected through a resistor R551 to an amplifiercomprising an operational amplifier O503. The amplifier has anamplification degree that is variable by variations of a variableresistor R564 for amplifying the mirror mark (ODF) and saturating theother components, as shown in FIG. 11C. The output signal of operationalamplifier 0503 is supplied through a diode D515 to capacitor C527. DiodeD515 is forward biased to charge capacitor C527 when the voltage at theoutput of operational amplifier 0503 exceeds the voltage acrosscapacitor C527. Since the mirror mark (ODF) has a level that is higherthan the other components of the played back RF signal, diode D515 isreverse biased so that capacitor C527 is discharged through resistorR555 when no mirror mark is played back. Therefore, the voltage acrosscapacitor C527 has a high level when a mirror mark is played back and itdecreases gradually through the resistor R555 into a sawtooth voltage,as shown in FIG. 11E. The voltage across capacitor C527 is applied viaFET Q509 to a differentiating circuit that comprises a capacitor C528and resistors R557 and R558. FIG. 11F shows a waveform of a signalproduced at the output of the differentiating circuit. The output of thedifferentiating circuit is coupled to a positive input of operationalamplifier 0504, which has a reference voltage V1 set by a variableresistor R560, connected to a negative input. Operational amplifier 0504forms a comparator along with transistor Q510, resistor R559, diode D525and capacitor C529. This comparator compares the differentiated signalwith a reference value V1 to produce a pulse having a predeterminedpulse width, as shown in FIG. 11G, each time a mirror mark is playedback. The pulse width is determined by capacitor C529 and resistor R559.Capacitor C527, which forms a peak hold circuit, serves to eliminatenoises, as shown in FIG. 11D, which may be superimposed on thedifferentiated signal.

The reading circuit (not shown) connected to output terminal 33 producesa sector pulse, shown in FIG. 12B, at the end of the sector mark (SM)included in the pre-format signal, shown in FIG. 12A. The sector pulsesignal is applied to input terminal 39 connected to a first monostablemultivibrator M501. The first monostable multivibrator M501 produces apulse having a predetermined pulse width in response to the sectorpulse. The pulse width is somewhat shorter than the time durationbetween the time at which a sector pulse occurs and the time at which amirror mark (ODF) occurs, as shown in FIG. 12C. The output of the firstmonostable multivibrator M501 is coupled to trigger a second monostablemultivibrator M502. The second monostable multivibrator M502 produces apulse with a logic level L having a predetermined width in response to atrailing edge of a pulse produced from the first monostablemultivibrator M501. The pulse width is somewhat longer than the durationof the mirror mark, as shown in FIG. 12D. Thus, the pulse produced fromthe second monostable multivibrator M502 forms a window for thedetection of a mirror mark.

The output of the second monostable multivibrator M502 is coupledthrough diode D511 to an emitter of a transistor Q515. Transistor Q515has an emitter connected through resistor R546 to a specific voltage anda collector connected to a drain of FET Q507. The emitter of transistorQ515 is also connected through inverter I501, having a structure shownin FIG. 6A, to input terminal 35, to which an -ON TRK signal is appliedfrom computer 50. The -ON TRK signal has a low level when a trackingservo is performed and a HIGH level when the tracking servo control issuspended. Inverter I501 produces a LOW output in response to a HIGHinput and the output thereof is OPEN in response to a LOW input. Theoutput of inverter I501 is coupled to the emitter of transistor Q515.Transistor Q515 has a base connected to a voltage divider includingresistors R545 and R547 and a collector connected to the drain of FETQ507.

When inverter I501 produces a LOW output indicating that the trackingservo control is suspended, transistor Q515 is turned OFF so that thesecond monostable multivibrator M502 has no function on the FET Q507. Onthe other hand, when the tracking servo control is resumed, the outputof inverter I501 is OPEN. Therefore, each time the second monostablemultivibrator M502 produces a negative mirror mark window pulse, asshown in FIG. 12D at the timing in compliance with the presence of amirror mark included in the RF signal shown in FIG. 12A, diode D511 isforward biased to turn OFF transistor Q515. As a result, FET Q507 isreleased from a saturated condition, which continues in the absence ofthe mirror mark window pulse, to permit the mirror mark detectingoperation in the same manner as described above. Operational amplifierO503, capacitor C527 of the peak hold circuit, the differentiatingcircuit comprising capacitor C528, and the operational amplifier O504produce the output signals shown in FIGS. 13C to 13F, respectively.

It is to be noted that the sector pulses can be detected with a highaccuracy when the tracking servo control is ON, whereas the detectionaccuracy of the sector pulses is reduced when the tracking servo controlis suspended. For this reason, the detection of the mirror marks isperformed only when the mirror window pulse is generated where thetracking servo control is ON. Where the tracking servo control issuspended, however, the mirror mark detection circuit is alwaysactivated.

The RF signal, shown in FIG. 8A, which is inputted from the inputterminal 31 is inverted and amplified by the operational amplifier 0501,as shown in FIG. 8B. Furthermore, the output signal of operationalamplifier 0501 is inverted and amplified by an amplifier that includestransistors Q516 and Q517, and applied to a peak hold circuit comprisingcapacitor C510, resistors R519 and R520, and diodes D505, D506 and D507.When the voltage at the output of transistor Q516 exceeds the voltageacross the capacitor C510, the diode D505 is forward biased to chargethe capacitor C510 with the output of transistor Q516. When the voltageacross capacitor C510 exceeds the voltage at the output of transistorQ516, diode D505 is reverse biased to block the transmission of thesignal to the capacitor C510. At this time, diode D506 remains turnedOFF and capacitor C501 is discharged through the series circuit of diodeD507 and resistor R520 for the next peak hold cycle. Thus, the peak ofthe signal outputted from the transistor Q516 is held by the capacitorC510 as shown in FIG. 8C and the charged voltage of the capacitor C510is applied to a gate of FET Q504.

FET Q504 amplifies the signal inputted to its gate and outputs theamplified signal to a low pass filter that includes resistors R517 andR518 and a capacitor C511. This low pass filter is effective to filterout the high-frequency noises which may be superimposed on the peakenvelope signal during the peak hold operation and also to limit thefrequency band (about 10 MHz) of the RF signal to a servo band (forexample, 50 KHz). The output of the low pass filter is coupled to apositive terminal of an operational amplifier 0502. The operationalamplifier 0502 forms a clamping circuit along with capacitor 512, diodeD508 and transistors Q505 and Q506 for clamping the input signal to areference voltage VE, determined by a variable resistor R535. Thereference voltage (clamp level) VE is applied to an output terminal 37and also to the base of transistor Q505.

FIG. 15B shows the waveform of an input signal applied to the positiveinput of the operational amplifier 0502 in FIG. 7. As is apparent fromthe waveform, the level drops thereof are compensated. FIG. 15C showsthe waveform of the output signal produced at the output of theoperational amplifier 0502. When the input signal increases and thus theoutput signal increases, the voltage at the emitter of the transistorQ505 increases. As a result, the current flow from resistor R533 totransistor Q505 decreases and the current flow from resistor R533 totransistor Q506 increases. Consequently, the voltage applied to thenegative input of the operational amplifier 0502 increases to decreasethe voltage at the output of operational amplifier O502. The voltage VEapplied to the base of the transistor Q505 is higher by a voltage Vbethan the voltage at the emitter thereof. The voltage at the output ofthe operational amplifier O502 is higher by the forward voltage of thediode D508 than the voltage at the emitter of the transistor Q505. Sincethe voltage Vbe is substantially equal to the forward voltage of diodeD508, operational amplifier O502 produces an output signal having peaksclamped to the voltage VE which is set at the base of the transistorQ505, as shown in FIG. 15C. The voltage, which appears at the output ofthe operational amplifier O502, is applied as a +TCR signal to outputterminal 34. It is to be noted that the +TCR signal has a level that iscompensated for its level drop which is caused when the optical headpasses one of the recorded areas of the optical disk, as shown in FIG.15C. The +TCR signal is applied to terminal 34 in FIG. 3.

The junction of diode D506 and resistor R519 is connected to a collectorof a transistor Q511, an emitter of which is connected to a specificvoltage and the base of which is connected to the output of operationalamplifier O504. When operational amplifier Q504 produces a HIGH levelpulse, shown in FIGS. 11G and 13F, indicating the occurrence of a mirrormark, transistor Q511 is turned OFF to turn ON diode D506, which hasbeen revere biased. As a result, capacitor C510 is discharged at a highrate through the series circuit of diode D506 and resistor R519 when amirror mark is detected. This is effective to inhibit capacitor C510from being charged with the voltage resulting from the mirror mark.

It is to be noted that the peak hold circuit may be arranged as a bottomhold circuit.

FIGS. 14A to 14D show the waveforms of various signals on a reduced timescale. FIG. 14A shows the waveform of the RF signal inputted to inputterminal 31 when the focusing servo control is ON and the tracking servocontrol is OFF. The RF signal includes track cross signals, producedwhen the optical head traverses tracks, and the VFO signals. FIG. 14Bshows the waveform of the RF signal which is outputted from theoperational amplifier O501 and includes the compensated VFO signals. Thewaveform of the signal produced at the output of the peak hold circuitC510 is as shown in FIG. 14C and the waveform of the signal produced atthe low pass filter, including capacitor C511, is an upper envelope ofthe output signal of the peak hold circuit C510, is shown in FIG. 14D.

If there was no means for preventing capacitor C510 from being chargedwith the voltage resulting from the mirror mark (ODF), the waveforms ofthe output signals of capacitors C510 and C511 will be as shown by thebroken line in FIGS. 14C and 14D, respectively. Namely, the signaloutputted from capacitor C511 includes the components of mirror markswhich are not included in a signal (TA+TB) which has a relatively narrowfrequency band (at most 50 KHz). As described below, in this invention,a control signal, indicative of level drops in compliance with therecorded areas, is produced from the signal (TA+TB) and a signal inwhich the level drops of the VFO signals are compensated. Accordingly,if the charging of capacitor C510 by the mirror mark was not prevented,the mirror mark components are included in the control signal. This isprevented in this invention because the time constant in the peak holdcircuit is extremely reduced at the time of occurrence of the mirrormarks.

The junction of diode D507 and resistor R520 is connected through aninverter I502 to input terminal 36, to which a -WGM signal is appliedfrom the computer 50. The -WGM signal has a LOW level when the automaticgain control apparatus is operating in a recording mode and a HIGH levelwhen it is in a playback mode (or a selected one of modes other than therecording mode). Inverter I502 has a structure shown in FIG. 6B and itproduces a HIGH level output in response to a LOW level input and theoutput thereof is OPEN in response to a HIGH level input. Consequently,diode D507 is maintained reverse biased to prevent capacitor C510 frombeing discharged during a recording mode where the signal inputted tocapacitor C510 does not include the signal played back from the opticaldisk.

Returning to FIG. 3, terminal 17 to which the TA signal is applied iscoupled through resistor R135 to a positive terminal of an operationalamplifier O104. Terminal 18, to which the TB signal is applied, iscoupled through resistor R136 to the positive input of the operationalamplifier O104. The operational amplifier O104 serves as an adder forproducing a signal (TA+TB) which is indicative of the sum of the TA andTB signals. It is to be noted that the signal (TA+TB) includes trackcross signals produced when the optical head traverses the tracks andlevel drops produced when the optical head passes one of the recordedareas having data recorded thereon. This signal (TA+TB) is appliedthrough resistor R110 to the positive terminal of an operationalamplifier O102, having a negative input coupled to a reference voltagesource that includes resistors R111 and R112. The operational amplifierO102 serves as a comparator to compare the added signal (TA+TB) with areference voltage to produce a Focus Zone (F+) signal indicative of afocusing zone. The output of the operational amplifier O102 is coupledto an output terminal 41.

Signal (TA+TB) is also applied from the output of the operationalamplifier O104 through resistor R108 to the second input 14 of the firstintegrated circuit IC101. The first integrated circuit IC101 divides theerror signal FB-FA applied to the first input I1 thereof by the signal(TA+TB) to produce an output signal that is indicative of the resultingquotient (FB-FA)/(TA+TB). Output terminal I3 of the first integratedcircuit IC101 is coupled to an amplifier including an operationalamplifier O103 for amplifying the signal fed thereto from the firstintegrated circuit IC101 to produce a focusing error signal F. Thefocusing error signal is applied to output terminal 42 for connection tothe focusing servo control circuit.

Terminal 37, to which the Clamp Ref signal produced by the resistorsR535 and R534 shown in FIG. 7 is applied through a resistor R181 to anegative terminal of an operational amplifier O107, has a positive inputcoupled through resistor R183 to terminal 34, to which the +TCR signalproduced from the operational amplifier O502, shown in FIG. 7 isapplied. The operational amplifier O107 serves as a differentialamplifier for producing a differential signal (+TCR-VE) indicative of adifference of the Clamp Ref signal VE from the +TCR signal. It is to benoted that the differential signal (+TCR-VE) includes the track crosssignals produced when the the optical head traverses the tracks but notthe level drop in compliance with the recorded area. The output of theoperational amplifier O107 is coupled to a drain of an FET Q101 having asource coupled through resistors R186 and R171 to a negative terminal ofan operational amplifier O105. The gate of the FET Q101 is coupledthrough inverters I102 and I103 to terminal 35, to which an -ON TRKsignal is applied from the computer 50. Inverters I102 and I103 havestructures as shown in FIGS. 6A and 6B, respectively. Inverter I102produces a LOW level output in response to a HIGH level input and theoutput thereof is OPEN in responsive to a LOW level input. Inverter I103produces a HIGH level output in response to a LOW level input and theoutput thereof is OPEN in response to a HIGH level input.

As previously, the -ON TRK signal has a low level when the trackingservo control is ON and a HIGH level when the tracking servo control isOFF. When the tracking servo control is OFF, FET Q101 is turned ON, tocouple the output of operational amplifier O107 to the negative input ofoperational amplifier O105, while a positive input is coupled to theoutput of operational amplifier O104. Operational amplifier O105 servesas a differential amplifier for producing a differential signal that isindicative of a signal difference {(TA+TB)-(+TCR-VE) } of the signal(+TCR-VE) from the signal (TA+TB). Since the signal (TA+TB) includes thetrack cross signals produced when the optical head traverses the tracksand the level drops produced when the playback is performed in therecorded areas having data recorded thereon, whereas the signal(+TCR-VE) includes only the track cross signals, the differential signal{(TA+TB)-(+TCR-VE)} produced at the output of operational amplifier O105includes only the level drop components. The output of operationalamplifier O105 is coupled through resistor R142 to the second input I4of the second integrated circuit IC102. The second integrated circuitIC102 divides the error signal (TB-TA) applied to the first input I1thereof from operational amplifier O108 by the differential signal{(TA+TB)-(+TCR-VE)} applied to the second input I4 thereof fromoperational amplifier O105 to produce an output signal that isindicative of a resulting quotient {(TB-TA)/((TA+TB)-(TCR-VE))}. Thesignal is outputted from output terminal I3 of the second integratedcircuit IC102 to a negative input of operational amplifier O106 thatforms a negative amplifier along with resistors R149, R151 and R155, andthe output signal of the negative amplifier is supplied as a trackingerror signal T- to output terminal 43, for connection to the trackingservo control circuit.

The operation of the gain control apparatus of this invention will bedescribed further with reference to FIGS. 15A to 15F. When the opticalhead is moving at a high rate in the radial direction of the opticaldisk with the focusing servo control being ON and with the trackingservo control being OFF, the reproduced RF signal has a level that isincreased (track cross component) each time the optical head traverses atrack and a level that is decreased each time the optical head passesone of the recorded areas including preformat areas having pre-formatdata recorded thereon. FIG. 15A shows the waveform of the signal (TA+TB)produced at the output of the operational amplifier O104. The signal(TA+TB) includes no playback data but the level drop component incompliance with the recorded areas, because the amplifier including theoperational amplifier O104 has a narrow freqency band as describedpreviously.

The differential amplifier including the operational amplifier O107subtracts the Clamp Ref signal (DC component VE) from, the +TCR signalto produce an output signal (TCR-VE), as shown in FIG. 15D. This outputsignal contains only the track cross components. On the other hand, thesignal (TA+TB) outputted from operational amplifier O104 contains thetrack cross components and the level drop components in compliance withthe recorded areas, as shown in FIG. 15A. The differential amplifierincluding the operational amplifier O105 subtracts the signal (TCR-VE)from the signal (TA+TB) to produce a control signal {(TA+TB)-(+TCR-VE)}which includes only the DC components produced when the optical headpasses the recorded areas, as shown in FIG. 15E. Although the referencevoltage VE is used to set the DC bias for the +TCR signal at apredetermined value, it is to be noted that the reference voltage VE maybe set at zero. In this case, the operational amplifier O107 is removedand terminal 34 is directly coupled to the drain of FET Q101.

The control signal {(TA-TB)-(+TCR-VE)} is applied to the second inputterminal I4 of the second integrated circuit IC102 which receives theerror signal (TB-TA) at the first input I1 thereof. The waveform of theerror signal (TB-TA) is indicated by the broken curve in FIG. 15F. Thesecond integrated circuit IC102 divides the error signal (TB-TA) by thecontrol signal {(TA+TB)-(+TCR-VE)} to produce an output signalindicative of the resulting quotient at its output I3. This outputsignal increases as the control signal fed from the output ofoperational amplifier O105 decrease and vice versa. The output signal isapplied to the amplifier including the operational amplifier O106 andamplified by it to produce a tracking error signal T-. The trackingerror signal T-, indicated by the solid curve in FIG. 15F, is applied tothe output terminal 43 for connection to the tracking servo controlcircuit (not shown).

This operation is performed not only when the optical head is moving ata high rate from one track to another during a seek or jump mode, butalso when the optical head traverses one or more tracks before thetracking servo control is ON.

When the tracking servo control is resumed for a normalrecording/playback operation, the -ON TRK signal applied to terminal 35in FIG. 3 is changed to its LOW level. As a result, the FET Q101 isturned OFF to block the transmission of the differential signal(+TCR-VE) to the negative input of the differential amplifier O105.Consequently, operational amplifier O105 produces a control signalcorresponding to the signal (TA+TB) applied thereto from operationalamplifier O104. Since the tracking servo control is ON, the signal(TA+TB) includes no track cross component but the level drop incompliance with the recorded areas. Therefore, the control signal(TA+TB) is applied from the operational amplifier O105 to the secondinput I4 of the second integrated circuit IC102 in the same way as inthe case described above.

Although in the embodiment described above, the control signal producedfrom the output of the differential amplifier including the operationalamplifier O105 is used only to compensate the tracking error signal, itis to be noted, of course, that the control signal may also be used tocompensate the focusing error signal. In this case, the output of thedifferential amplifier including the operational amplifier O105 isconnected through resistor R108 to the second input I4 of the firstintegrated circuit IC101, instead of the connection of the output of theoperational amplifier O104.

What is claimed is:
 1. An automatic gain control apparatus in arecording medium playback system which has a head for playing back arecording medium with tracks on which information is recorded and forgenerating a playback signal including a signal having a predeterminedfrequency, said apparatus comprising:means for generating a trackingerror signal for a tracking servo to control a relative position of saidhead with respect to one of said tracks, said tracking servo, when in anON state, controlling the relative position of said head in a trackingdirection; control means for modifying an amplitude level of saidtracking error signal in accordance with a control signal; means forgenerating a compensated playback signal by modifying an amplitude levelof said signal; and means for generating said control signal from saidcompensated playback signal when said tracking servo is in an OFF state.2. An automatic gain control apparatus as claimed in claim 1,whereinsaid control signal generating means further generates said controlsignal from said playback signal when said tracking servo is ON.
 3. Anautomatic gain control apparatus as claimed in claim 1,wherein saidcontrol means includes means for dividing said tracking error signal bysaid control signal.
 4. An automatic gain control apparatus as claimedin claim 1,wherein said apparatus further comprises means for producingfirst and second tracking signals, said control signal generating meansincludes means for adding said first and second tracking signals toproduce said control signal, and said tracking error signal is producedby subtracting said first tracking signal from said second trackingsignal.
 5. An automatic gain control apparatus as claimed in claim4,wherein said control signal generating means comprises:meansresponsive to said playback signal for generating a second signalincluding a track cross component produced when said head is traversingsaid tracks, and means for producing said control signal by subtractingsaid second signal from a sum signal of said first and second trackingsignals to cancel said track cross components.
 6. An automatic gaincontrol apparatus as claimed in claim 1,wherein said apparatus furthercomprises: means for generating a focusing error signal for focusingservo to control a relative position of said head with respect to saidrecording medium by controlling the position of said head in a focusingdirection; and means for modifying an amplitude level of said focusingerror signal in response to said control signal.
 7. An automatic gaincontrol apparatus as claimed in claim 1,wherein said recording mediumcomprises a disk and said compensated playback signal generating meanscomprises: means for extracting said signal from said playback signal;means for producing a gain control signal having an amplitude levelcorresponding to said head position in a radial direction of said disk;means for amplifying said signal extracted by said extracting means atan amplification degree determined by said gain control signal level;and means for adding an output of said amplifying means to said playbacksignal.
 8. An automatic gain control apparatus as claimed in claim5,wherein said compensated signal generating means further comprises:means for extracting said signal from said playback signal, means foramplifying said signal extracted by said extracting means by apredetermined amplification degree, and means for adding an output ofsaid amplifying means to said playback signal; and further wherein saidcontrol signal generating means further comprises means for holding apeak level of an output of said amplifying means; and low-pass filtermeans for passing a low frequency component of an output of said peakhold means.
 9. An automatic gain control apparatus as claimed in claim8,wherein said compensated playback signal generating means furthercomprises means for clamping an output of said low-pass filter means ata predetermined amplitude level.
 10. An automatic gain control apparatusas claimed in claim 8,wherein said disk comprises an optical disk havingtracks, each track including a plurality of sectors, each sector havinga pre-format area preceding a user area, said pre-format area havingpre-format data recorded thereon, wherein said playback signal includesa pre-format signal in compliance with said pre-format data, saidpre-format data including a sector mark, a mirror mark and clockinformation and address information signals which are repeated apredetermined number of times alternatively between said sector mark andsaid mirror mark, said mirror mark being recorded so as to have anamplitude level when played back that is higher than any other signalsincluded in said pre-format signal, said clock information signals eachhaving a frequency that is higher than frequencies of any other signalsincluded in said pre-format signal for use in playing back saidrespective address information signals succeeded thereto.
 11. Anautomatic gain control apparatus as claimed in claim 10,wherein saidclock information signal comprises said signal.
 12. An automatic gaincontrol apparatus as claimed in claim 5,wherein said control signalgenerating means further comprises switching means for transmitting saidsecond signal to said control signal producing means when said trackingservo is OFF and inhibiting the transmission of said second signal tosaid control signal producing means when said tracking servo is ON. 13.An automatic gain control apparatus as claimed in claim 10.wherein saidapparatus further comprises: means responsive to said mirror markincluded in said playback signal for producing a mirror mark detectionsignal; and means responsive to said mirror mark detection signal forcontrolling the operation of said peak hold means so that the holding ofsaid peak level of said playback signal in compliance with said mirrormark is inhibited.
 14. An automatic gain control apparatus as claimed inclaim 13,wherein said means responsive to said mirror mark furthercomprises: means for eliminating said clock information signal from anoutput of said amplifying means; second amplifying means for amplifyingan output of said eliminating means; second peak hold means for holdingpeak level output of said second amplifying means; differentiating meansfor differentiating an output of said second peak hold means; and meansfor comparing an output of said differentiating means with apredetermined reference level.
 15. An automatic gain control apparatusas claimed in claim 14,wherein said means responsive to said mirror markfurther comprises: means for generating a mirror mark window pulse whichis produced concurrently with an occurrence of said mirror mark; andmeans responsive to an output of said mirror mark window pulse so thatsaid mirror mark detection is performed at the timing only when saidmirror mark window pulse is generated.
 16. An automatic gain controlapparatus as claimed in claim 15,wherein said means for generating amirror mark window pulse comprises: a first multivibrator which istriggered by a signal generated in compliance with said sector mark toproduce a pulse with a predetermined width; and a second multivibratorwhich is triggered by said pulse outputted from said first multivibratorto produce said mirror mark window pulse.